1. Field of the Invention
The present invention relates to a chopper type comparator used for an analog-to-digital converter, etc.
2. Description of the Related Art
FIG. 2 is a circuit diagram of a conventional chopper type comparator disclosed in Japanese Patent Application Kokai No. 2005-57717.
The chopper type comparator includes: a switch 1 for connecting and disconnecting between a terminal to which a reference voltage VREF is supplied and a node NA; and a switch 2 for connecting and disconnecting between a terminal to which an analog signal VIN is supplied and the node NA. The node NA is connected through a capacitor 3 to an input (a node NB) of an inverter circuit 4. An output (a node NC) of the inverter circuit 4 is connected through a capacitor 5 to an input (a node ND) of an inverter circuit 6. Further, the input NB of the inverter circuit 4 is connected through a switch 7 to the output NC of the inverter circuit 4. The input ND of the inverter circuit 6 is connected through a switch 8 to an output terminal of the inverter circuit 6. An output signal indicating a comparison result OUT is output from the output terminal of the inverter circuit 6.
The inverter circuits 4 and 6 have the same configuration, in each of which two gated inverters are connected in parallel. A gated inverter is an inverter having a current-controlling transistor inserted in series with source and ground lines thereof. The inverter circuits 4 and 6 are arranged so that the current-controlling transistors of the gated inverters are controlled in their conducting states by control voltages X and Y.
The typical operation of the chopper type comparator is as follows.
In an input period of the analog signal VIN, the switch 1 stays OFF, and the switches 2, 7 and 8 become ON. Thus, the potential of the node NA becomes VIN. The potentials of the nodes NB and NC become VT4, VT4 being a threshold voltage of the inverter circuit 4. Further, the potential of the node ND becomes VT6, which is a threshold voltage of the inverter circuit 6. In this case, charges Q3 and Q5 stored in the capacitors 3 and 5 are given by:Q3=C3×(VIN−VT4)  (1)Q5=C5×(VT4−VT6)  (2)where capacitances of the capacitors 3 and 5 are denoted by C3 and C5, respectively.
Then, the switch 2 is turned off. After that, the switches 7 and 8 are turned off, and further the switch 1 is turned on. At this time, when the potential of the node NB is represented by VB, the charge Q3a of the capacitor 3 is given by:Q3a=C3×(VREF−VB)  (3)
Here, Q3a=Q3, since the charge of the capacitor 3 does not change. Therefore, from the equations (1) and (3), the potential VB can be expressed as follows.VB=VREF−VIN+VT4  (4)
When the voltage gain of the inverter circuit 4 is represented by A4, the potential VC of the node NC is given by:VC=A4−(VREF−VIN)+VT4  (5)
Further, when the potential of the node ND is represented by VD, the charge Q5a of the capacitor 5 is given by:Q5a=C5×(VD−VD)  (6)
Here, Q5a=Q5, since the charge of the capacitor 5 does not change. Therefore, from the equations (2), (5) and (6), the potential VD becomes as follows.VD=A4×(VREF−VIN)+VT6  (7)
Further, when the voltage gain of the inverter circuit 6 is represented by A6, an output signal OUT output from the inverter circuit 6 is given by:OUT=A4×A6×(VREF−VIN)+VT6  (8)
Accordingly, the chopper type comparator operates as an amplifier that amplifies a potential difference (VREF−VIN) between the reference voltage VREF and analog signal VIN by a factor of A4×A6. Then, the output signal OUT which is the result of comparison can be obtained.
Meanwhile, the chopper type comparator shown in FIG. 2 incorporates the inverter circuits 4 and 6 which use gated inverters. Accordingly, the voltage gains A4 and A6 of the inverter circuits 4 and 6 can be arbitrarily controlled by control voltages X and Y. Thus, the following are made possible: to make a high-speed comparison using an increased gain when a high resolution is required; and to reduce power consumption by decreasing the gain when a high resolution is not required.
However, the above-described chopper type comparator has had a problem such that through current of the inverter circuits is increased when operating at a high speed.